Images, such as images captured by digital cameras or computer generated images contain color information defining the color characteristics of each pixel or a group of pixels. Such color information can be represented as color components defining color in a color space. Color components can, for example, represent the intensity of a color as a measure of luma brightness and chroma color deviations from neutral grayscale color toward blue or red (YUV) for or as a measure of red, green, or blue light component intensity (RGB). A YUV model generally defines a color space in terms of one luma (Y) and two chrominance (UV) components. Generally Y represents the luma component (brightness) and U and V are the chrominance (color) or chroma components.
The XYZ color space is an international standard developed by the CIE (Commission Internationale de l'Eclairage). This color model is based on three primaries, XYZ, and all visible colors are represented using positive values of X, Y, and Z. The Y primary is defined to match closely to luminance, while X and Z primaries give color information.
High dynamic range imaging (also referred to as HDR or HDRI) enables a greater dynamic range of luminance between light and dark areas of a scene, and a greater range of color gamut compared to traditional low dynamic range (LDR) images. This is achieved in HDR imaging by extending the signal representation to a wider dynamic range in order to provide high signal accuracy across the entire range. In HDR images, color component values of pixels are usually represented with a greater number of bits (for example from 16 bits to 64 bits) including in floating-point format (for example 32-bit or 16-bit for each component, namely float or half-float), the most popular format being openEXR half-float format (16-bit per RGB component, i.e. 48 bits per pixel) or in integers with a long representation, typically at least 16 bits. Such ranges correspond to the natural sensitivity of the human visual system. In this way HDR images more accurately represent the wide range of luminance and color found in real scenes thereby providing more realistic representations of the scene.
In order to reduce the amount of memory required for storage of color information, and in particular of HDR color information, and to reduce bandwidth usage in transmission, many techniques exist for compressing color information, of image data into smaller, more manageable data sizes.
For example, the SMPTE 2085 standard defines YDzDx color-difference encoding to be used for high dynamic range XYZ signals. Color information, represented by nonlinearly encoded X′, Y′ and Z′ values is transformed to Y′, D′z and D′x color-difference signals. The chroma signal D′z is defined as a weighted difference between components Z′ and Y′, and D′x is a weighted difference between color components X′ and Y′. The luminance signal Y′ is unaltered by the transformation. The SMPTE 2085 standard defines the conversion from X′Y′Z′ to Y′D′ZD′X and the conversion from Y′D′ZD′X to X′Y′Z′
The dynamic of the color difference components D′z and D′x depends on the color space used for the content. For instance, in ITU Rec709, D′x varies within the range [−0.135; 0.131] whereas the excursion range is [−0.496; 0.147] when ITU Rec2020 is considered. Thus, the useful dynamic is about 65% of the full range in ITU Rec2020 but only 27% in ITU Rec709. The reduced input data can lead to the introduction of more quantization noise at the encoder input.
The present invention has been devised with the foregoing in mind.